A Newly Designed Pulsed Valve for Molecular Beam Experiments

1 A Newly Designed Pulsed Valve for Molecular Beam Experiments 

To study catalytic processes of clusters on surfaces with high sensitivity, a newly designed pulsed valve with excellent pulse-to-pulse stability, in combination with absolutely calibrated mass spectrometry, was used to determine turn-over frequencies [75]. 

After ejecting the CO molecule to the substrate at a constant isotropic pressure of O2 (5 X 10 mbar), the resulting product molecules (CO2) were detected by an absolutely calibrated quadrupole mass spectrometer. The differentially pumped, computer-controlled mass spectrometer (Balzers QMG 421) is mounted in line-of-sight with the sample. At the entrance, near the ionizer of the quadrupole mass spectrometer, a skimmer with an opening of 3 mm is mounted. The skimmer is biased to —150 V in order to prevent electron-stimulated desorption induced by emitting electrons from the ionizer of the mass spectrometer. After amplifying 

More important is the evaluation of the integral of the CO2 pulse. The mass spectrometer was calibrated by measuring the reflected CO from the MgO substrate of a known pulse from the piezo-electric valve. The error of this measurement is estimated to be about 10%. As the cluster density on the surface (see above) is also known, turn-over frequencies (TOF) for the oxidation of CO can be extracted by simply calculating the ratio of the total number of CO2 formed and the number of clusters on the surface times the pulse duration. 

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